The invention relates to an illumination mode switching circuit for an automatic electronic flash, and more particularly, to such a switching circuit which utilizes a single changeover switch to permit a selection among a plurality of different illumination modes available in an automatic electronic flash.
As is well recognized, an automatic electronic flash is mounted on a camera when in use includes a flash discharge tube, and the emission of light therefrom is automatically controlled in accordance with an output from a photoelectric transducer element which is provided within the electronic flash for purposes of photometry, when the latter is in its automatic illumination control mode. By changing a mode selecting, changeover switch, the electronic flash is capable of selecting any one of other modes, including a full illumination mode in which the electronic flash is permitted to provide a full emission of light, and a manual illumination control mode in which the emission of light from the electronic flash is controlled to a value which is one-half or one-quarter the full amount of emission, for example. In addition, when the automatic illumination control mode is selected, a number of automatic illumination control levels can be chosen in accordance with a diaphragm value of the taking lens of a camera. Furthermore, with an automatic electronic flash which can be connected with a single lens reflex camera having a photometric circuit of TTL (through the taking lens) photometry type, a TTL automatic illumination control mode is available in which the emission of light from the flash discharge tube is controlled in accordance with an output from the photoelectric transducer element provided on the part of the camera.
FIG. 1 is a block diagram illustrating the fundamental electrical circuit of an automatic electronic flash. It essentially comprises a power supply circuit 1, flashlight emission circuit 2 including a flash discharge tube, not shown, and a flashlight or illumination interrupting circuit 3 which operates to interrupt the emission of light from the flash discharge tube at a suitable time. The flashlight emission circuit 2 causes a flashlight to be emitted from the flash discharge tube in synchronized relationship with the full opening of a shutter in response to the closure of synchro contact X which is disposed within the camera. The illumination interrupting circuit 3 is connected with an illumination mode switching circuit 6, which includes a photometric integrating circuit 4 which is adapted to provide an automatic illumination control, and a time constant circuit 5 which is used to provide a manual illumination control. The integrating circuit 4 includes a photometric, photoelectric transducer element, not shown, which provides an automatic illumination control, and a changeover switch S3 which is used to establish an automatic illumination control level providing a proper exposure level when taking pictures in an automatic illumination control mode, in accordance with a diaphragm value, for example, F8, F5.6, F4 or the like of the taking lens of the camera. Integrating circuit 4 is connected to a fixed contact S2a of an illumination mode selecting, changeover switch S2. The time constant circuit 5 is formed by a well known capacitor-resistor timer circuit which controls the duration of illumination from the flash discharge tube. The circuit 5 is connected to another fixed contact S2c of the changeover switch S2. The changeover switch S2 includes a further fixed contact S2b which is left without circuit connection. The switch also includes a movable contact S2d, and when the movable contact is connected to the fixed contact S2b, the photometric integrating circuit 4 and the time constant circuit 5 are disabled. A TTL automatic illumination control mode selecting, changeover switch S1 is connected between the movable contact S2d of the changeover switch S2 and the illumination interrupting circuit 3, and includes a movable contact which may be thrown to either fixed contact S1a or another fixed contact S1b. When the movable contact is thrown to the fixed contact S1a, the illumination interrupting circuit 3 is connected to the changeover switch S2 while when the movable contact is thrown to the other fixed contact S1b, the illumination interrupting circuit 3 is connected to a photometric integrating circuit 7 which is provided within the camera to provide a TTL automatic illumination control.
In operation, the flash discharge tube emits flashlight, which is directed to an object being photographed. Reflective light L from the object is determined by the photometric integrating circuit 4. The changeover switch S2 which is used to select a particular illumination mode can be operated to select either one of an automatic illumination control mode in which the emission of light from the flash discharge tube is interrupted whenever a proper exposure is reached, a full illumination mode which permits a full emission of light from the tube, and a manual illumination control mode in which the emission of light from the tube is interrupted after a given time interval. In addition, the changeover switch S1 may be operated to select the TTL automatic illumination control mode. Additionally, the changeover switch S3 may be operated to choose one of automatic illumination control levels, thus permitting a proper exposure level to be selected when taking a picture in the automatic illumination control mode.
It will be seen that the automatic electronic flash mentioned above which permit a selection among a number of illumination modes utilizes a plurality of changeover switches S1-S3. In a conventional automatic electronic flash, the functioning of these changeover switches S1-S3 is provided by a multiple circuit, multiple contact rotary switch so that any one of the number of illumination modes can be selected through a single switch operation. However, the use of such a rotary switch presents difficulties in respect of the operational reliability, the cost of parts required and the ease of construction. As a consequence, the circuit arrangement of the illumination mode switching circuit becomes complex, accompanied by a complicated wiring to the switch and switch contacts, with the consequence that the overall reliability of the automatic electronic flash is reduced and its cost increased.
These disadvantages will be discussed in more detail with reference to FIG. 2 which illustrates one form of conventional illumination mode switching circuit. As shown, the circuit includes a ground potential bus E1 which is connected to a ground connection terminal G which is common with a camera. It also includes another bus E2 which is connected through a resistor R1 to a supply terminal Y which assumes a negative potential when the flash discharge tube emits flashlight. The photometric integrating circuit which provides an automatic illumination control, the time constant circuit utilized during a manual illumination control and illumination mode selecting, changeover switches are connected across the buses E1, E2. Specifically, connected across the buses E1, E2 are a parallel combination of a Zener diode ZD and a capacitor C1 which form together a power supply circuit for the illumination mode switching circuit, the photometric integrating circuit comprising a photometric, photoelectric transducer element PD, formed by a phototransistor, in series with a parallel combination of a resistor R2 and a capacitor C2, a variable resistor VR1 which is used to adjust a manual illumination control level, a series combination of a semi-fixed resistor VR2 and resistors R5, R6 and R7 which are used to establish automatic illumination control levels, and a time constant circuit comprising a parallel combination of a resistor R8 and capacitor C3 in series with a resistor R9.
The junction between the transducer element PD and resistor R2 is connected with fixed contacts a1-a3 of the illumination mode selecting, changeover switch SWa formed by a rotary switch. The switch SWa also includes a fixed contact a4 which is connected to the variable contact of the variable resistor VR1, and also includes other fixed contacts a5, a6 which are left without circuit connection. The movable contact a0 of the switch SWa is connected to the base of an NPN transistor Q1 through a resistor R3. The transistor Q1 operates to detect an illumination control level, and has its collector connected to the bus E1 through a resistor R4 and its emitter connected to the movable contact b0 of an automatic illumination control level selecting, changeover switch SWb which is formed by a rotary switch. The changeover switch SWb includes fixed contacts b1-b4, which are connected to the junction between the resistors R6, R7, the junction between the resistors R5, R6, the junction between the semi-fixed resistor VR2 and the resistor R5, and the junction between the resistors R8, R9, respectively. The switch SWb also includes fixed contacts b5, b6 which are left without circuit connection.
The collector of the transistor Q1 is connected to fixed contacts c1-c4 of a TTL automatic illumination control mode selecting, changeover switch SWc which is again formed by a rotary switch. The switch SWc inludes another fixed contact c5 which is left without circuit connection, and a further fixed contact c6 which is connected to a terminal, not shown, which is adapted to receive an automatic illumination control signal from the camera. The movable contact c0 of the changeover switch SWc is connected to an illumination control signal terminal T which is adapted to be connected to the illumination interrupting circuit.
Three changeover switches Swa, Swb and Swc are formed by a single rotary switch having three circuits and six contacts, and which constitutes an illumination mode selecting, changeover switch. By operating the switch to its successive positions, the movable contact of the changeover switch SWa can be thrown to successive fixed contacts a1-a6, the movable contact of the changeover switch SWb can be thrown to successive fixed contacts b1-b6, and the movable contact of the changeover switch SWc can be thrown to successive fixed contacts c1-c6, respectively.
Since the operation of the illumination mode switching circuit depends on the position of the respective changeover switches Swa, SWb and SWc, the operation will be described below with reference to individual positions of the changeover switches SWa, SWb and SWc.
(1) When the changeover switches SWa, SWb and SWc are thrown to respective fixed contacts a1, b1, c1, respectively:
In this instance, the base of the transistor Q1 is connected through the resistor R3 to the emitter of the transducer element PD, and the emitter of this transistor is connected to the junction between the resistors R6, R7 which are used to establish automatic illumination control levels. The collector of the transistor is connected to the terminal T which is in turn connected to the illumination interrupting circuit. In this manner, the photometric integrating circuit for automatic illumination control is formed.
When the synchro switch X is closed in response to the depression of a shutter button of camera, the flash discharge tube is allowed to emit flashlight, whereupon the terminal Y assumes a negative potential, thus back biasing the Zener diode ZD. In this manner, a given voltage is supplied across the buses E1, E2 for a given time interval. The flashlight from the discharge tube is reflected by an object being photographed, and impinges on the transducer element PD which is not activated for operation. Hence, the transducer element PD produces a photocurrent, which charges the integrating capacitor C2. When the voltage across the integrating capacitor C2 exceeds a threshold level of the transistor Q1 which depends on the magnitude of the resistor R7, and transistor Q1 is turned on, thus reducing or pulling the potential at the signal terminal T to the negative side to thereby activate the illumination interrupting circuit, thus interruping the emission of light from the flash discharge tube.
This provides an automatic illumination control mode in which a proper exposure is given when an increased value of diaphragm aperture, for example, F4, is chosen for the taking lens of the camera since the emission of light from the flash discharge tube is automatically interrupted when the transistor Q1 is turned on and since the threshold level of this transistor depends on the magnitude of the resistor R7.
(2) When the changeover switches SWa, SWb and SWc are thrown to respective fixed contacts a2, b2, c2, respectively:
The circuit connection is quite similar to that mentioned under paragraph (1) except that the level presetting resistors R6 and R7 are connected in series with the emitter of the transistor Q1. Hence, an automatic illumination control mode is provided in which a proper exposure is automatically established for an intermediate diaphragm value of the taking lens, for example. F5.6.
(3) When the changeover switches SWa, SWb and SWc are thrown to respective fixed contacts a3, b3 and c3, respectively:
The operation is similar to that mentioned under paragraph (1) except that level presetting resistors R5, R6 and R7 are connected in series with the emitter of the transistor Q1. Hence, an automatic illumination control mode is provided in which a proper exposure is established for a small diaphragm value of the taking lens, for example, F8.
(4) When the changeover switches SWa, SWb and SWc are thrown to respective fixed contacts a4, b4 and c4, respectively:
In this instance, the transistor Q1 has its base connected through the resistor R3 to the variable resistor VR1 which is used to adjust the manual illumination control level, its emitter connected to the junction between the resistors R8, R9 and its collector connected to the terminal T which is in turn connected to the illumination interrupting circuit. Hence, the photometric integrating circuit including the transducer element PD and whih provides the automatic illumination control is disconnected, while the time constant circuit is completed which provides the manual illumination control.
In this position of changeover switches, when the synchro switch X is closed in response to the depression of a shutter button of the camera, the flash discharge tube is allowed to emit flashlight, and an operating voltage is supplied across the buses E1, E2 for a given time interval. At the same time as the power supply across the buses E1, E2 is established, the time capacitor C3 begins to charge until the emitter potential of the transistor Q1 reduces below a given value over a given time interval, whereupon the transistor Q1 is turned on, driving the signal terminal to the negative to activate the illumination interrupting circuit, thus interrupting the emission of light from the flash discharge tube after it has emitted a given amount of flashlight. In this manner, the manual illumination control mode is provided in which a given proportion of light as referenced to the full emission of light from the flash discharge tube is emitted thereby.
(5) When the changeover switches SWa, SWb and SWc are thrown to respective fixed contacts a5, b5 and c5:
In this instance, the terminal T is disconnected from the illumination mode switching circuit, and hence the illumination interrupting circuit does not operate. Consequently, the emission of light from the discharge tube is not controlled, and a full emission of light is permitted. Accordingly, a manual full illumination mode is provided.
(6) When the changeover switches SWa, SWb and SWc are thrown to respective fixed contacts a6, b6 and c6:
In this instance, the illumination control signal terminal T is connected to a terminal which is adapted to receive an automatic illumination control signal from a photometric integrating circuit which is disposed in the camera to provide a TTL automatic illumination control. Accordingly, the illumination interrupting circuit operates in accordance with an automatic illumination control signal fed from the camera, thereby automatically interrupting the emission of light from the flash discharge tube. This provides the TTL automatic illumination control mode.
As discussed above, with the conventional illumination mode switching circuit shown in FIG. 2, a single rotary switch having three circuits and six contacts and which provides the function of the three changeover switches SWa, SWb, SWc may be operated to select one of the six illumination modes mentioned under paragraphs (1)-(6) above. However, the use of such rotary switch results in an increased cost which must be allowed to assure the operational reliability of switch itself. In addition, if the performance of the switch is assured, it remains to be complex. Such a rotary switch is very expensive itself, and requires a complex wiring therearound, whereby the cost of manufacturing an automatic electronic flash increases.